This application is related to a co-pending application entitled xe2x80x9cGRAPHICS PIPELINE INCLUDING COMBINER STAGESxe2x80x9d filed Mar. 22, 1999 naming David B. Kirk, Matthew Papakipos, Shaun Ho, Walter Donovan, and Curtis Priem as inventors, and issued as U.S. Pat. No. 6,333,744, and which is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to computer graphics, and more particularly to texture sampling in a computer graphics processing pipeline.
2. Background of the Invention
Recent advances in computer performance have enabled graphic systems to provide more realistic graphical images using personal computers and home video game computers. In such graphic systems, some procedure must be implemented to xe2x80x9crenderxe2x80x9d or draw graphic primitives to the screen of the system. A xe2x80x9cgraphic primitivexe2x80x9d is a basic component of a graphic picture, such as a polygon, e.g., a triangle, or a vector. All graphic pictures are formed with combinations of these graphic primitives. Many procedures may be utilized to perform graphic primitive rendering.
Early graphic systems displayed images representing objects having extremely smooth surfaces. That is, textures, bumps, scratches, or other surface features were not modeled. In order to improve the quality of the image, texture mapping was developed to model the complexity of real world surface images. In general, texture mapping is the mapping of an image or a function onto a surface in three dimensions. Texture mapping is a relatively efficient technique for creating the appearance of a complex image without the tedium and the high computational cost of rendering the actual three dimensional detail that might be found on a surface of an object.
Prior Art FIG. 1 illustrates a graphics pipeline with which texture mapping may be performed. As shown, included is a transform engine 100, a set-up module 102, a rasterizer 104, a texture math module 106, a level of detail (LOD) calculator 108, a texture fetch module 110, a texture filter 112, and a texture combination engine 114. It should be noted that the transform engine 100 and set-up module 102 need not necessarily be required in the graphics pipeline of a graphics integrated circuit.
During operation, the transform engine 100 may be used to perform scaling, rotation, and projection of a set of three dimensional vertices from their local or model coordinates to the two dimensional window that will be used to display the rendered object. The setup module 102 utilizes the world space coordinates provided for each triangle to determine the two dimensional coordinates at which those vertices are to appear on the two dimensional window. Prior Art FIG. 2 illustrates the coordinates 200 of the vertices 201 which define a triangle 202. If the vertices 201 of the triangle 202 are known in screen space, the pixel positions vary along scan lines within the triangle 202 in screen space and may be determined.
The setup module 102 and the rasterizer module 104 together use the three dimensional world coordinates to determine the position of each pixel contained inside each of the triangles. Prior Art FIG. 3 illustrates a plurality of pixels 300 identified within the triangle 202 in such a manner. The color values of the pixels in the triangle 202 vary from pixel to pixel in world space. During use, the setup module 102 and the rasterizer module 104 generate interpolated colors, depth and texture coordinates.
The setup module 102 and the rasterizer module 104 then feed the pixel texture coordinates to the texture math module 106 to determine the appropriate texture map colors. In particular, texture coordinates are generated that reference a texture map using texture coordinate interpolation which is commonly known to those of ordinary skill in the art. This is done for each of the pixels 300 identified in the triangle 202. Prior Art FIG. 3 illustrates texture coordinates 302 for the pixels 300 identified within the triangle 202.
Next, a LOD calculation is performed by the LOD calculator 108. Occasionally during rendering, one texel, or texture element, will correspond directly to a single pixel that is displayed on a monitor. In this situation the level of detail (LOD) is defined to be equal to zero (0) and the texel is neither magnified nor minified. However, the displayed image can be a magnified or minified representation of the texture map. If the texture map is magnified, multiple pixels will represent a single texel. A magnified texture map corresponds to a negative LOD value. If the texture map is minified, a single pixel represents multiple texels. A minified texture map corresponds to a positive LOD value. In general, the LOD value corresponds to the number of texels in the texture map xe2x80x98coveredxe2x80x99 by a single pixel.
The amount of detail stored in different LOD representations may be appreciated by drawing an analogy to the detail perceived by an observer while observing a texture map. For example, very little detail may be perceived by an observer while watching an automobile from a distance. On the other hand, several details such as doors, windows, mirrors will be perceived if the observer is sufficiently close to the automobile. A finer level LOD will include such additional details, and a courser LOD will not.
Once the appropriate level of detail of the texture map is selected based on the calculated LOD value, the texture coordinates generated by the texture math module 106 are used to fetch the appropriate texture map colors using the texture fetch module 110. These texture map colors are then filtered by the texture filter module 112. The combiner engine 114 combines together the various colors and textures fetched by the texture fetch module 110 and filtered by the texture filter module 112.
It is important to note that the pipeline described hereinabove has a linear topology. These and other simplistic non-linear pipelines only enable one texture fetch and texture calculation per rendering pass. This is a limited design that is static in nature. There is thus a need for a pipeline that allows for more dynamic texture fetches and shading calculations, and in particular, the ability for feeding filter results back to influence subsequent texture address calculations.
A system, method and article of manufacture are provided for interweaving shading calculations and texture retrieval operations during texture sampling in a graphics pipeline. First, a shading calculation is performed in order to generate output, i.e. colors or texture coordinates. Next, texture information is retrieved, and another shading calculation is performed using the texture information in order to generate additional output. Texture information may be retrieved and shading calculations may then be repeated as desired. Thereafter, the generated output may be combined. As such, the repeated texture information retrieval and shading calculations may be carried out in an iterative, programmable manner.
In one embodiment of the present invention, edge distances of a primitive may be calculated, and at least one of the shading calculations involves the edge distances. Further, the shading calculation may include the calculation of a plurality of weighted coefficients from the edge distances. As an option, such weighted coefficients may include barycentric weights which use parameter values of the primitive to perform parameter interpolation.
In another embodiment of the present invention, the texture information may include filtered texture color information. As an option, the filtered texture value may be used as texture coordinates for use in retrieving further texture information when the texture information retrieval is repeated. Further, the repeated shading calculation may also use the output in order to generate additional output.
In still another embodiment of the present invention, the output may include diffuse output colors, fog output values, specular output colors, depth output values, texture color output values, a level of detail (LOD) value, and/or a Z-slope value. As an option, the shading calculation may include the calculation of a level of detail (LOD) which may occur after the texture information is retrieved.
In order to accomplish the foregoing, a graphics pipeline system may be provided which includes a shading module for performing the shading calculation in order to generate output. Coupled to the shading module is a texture look-up module for retrieving texture information. Further, a feedback loop is coupled between an input and an output of the shading module for performing additional shading calculations using the texture information from the texture look-up module. Also included is a combiner module coupled to the output of the shading module for combining the output generated by the shading module. In one aspect of the present embodiment, at least a pair of texture look-up modules is coupled to a pair of shading modules which together constitute at least four logical modules.
These and other advantages of the present invention will become apparent upon reading the following detailed description and studying the various figures of the drawings.